Topology.close() calls self._update_servers() which will open new Monitor threads and connection pools for servers that are in the topology description but not in the self.servers dict: https://github.com/mongodb/mongo-python-driver/blob/3.8.0/pymongo/topology.py#L399

A simple repro is creating a client with connect=False and then calling close:

>>> client = MongoClient(connect=False)
>>> client._topology._servers
{}
>>> client.close()
>>> client._topology._servers
{('localhost', 27017): <pymongo.server.Server object at 0x10bd10e80>}
>>> client._topology._servers[('localhost', 27017)]._monitor._executor._thread
<weakproxy at 0x10bd35098 to Thread at 0x10bd10e48>

I think the simple fix is to remove the call to self._update_servers().